It's a movie geek's film to watch and to enjoy. But when I reached the end, I had no idea what to take from it.

Some of the ideas are amazing, the visuals, and the lines (I'm sorry, Dave. I'm afraid I can't do that) are brilliant. But why is it considered such a great film, while at the same time having no defined meaning or interpretation?

You're free to speculate as you wish about the philosophical and
allegorical meaning of the film—and such speculation is one indication
that it has succeeded in gripping the audience at a deep level—but I
don't want to spell out a verbal road map for 2001 that every viewer
will feel obligated to pursue or else fear he's missed the point.

And Clarke:

I still stand by this remark, which does not mean one can't enjoy the
movie completely the first time around. What I meant was, of course,
that because we were dealing with the mystery of the universe, and
with powers and forces greater than man's comprehension, then by
definition they could not be totally understandable. Yet there is at
least one logical structure—and sometimes more than one—behind
everything that happens on the screen in "2001", and the ending does
not consist of random enigmas, some simpleminded critics to the
contrary.

Freeman Dyson urged:

"After seeing Space Odyssey, I read Arthur Clarke's book. I found the
book gripping and intellectually satisfying, full of the tension and
clarity which the movie lacks. All the parts of the movie that are
vague and unintelligible, especially the beginning and the end, become
clear and convincing in the book. So I recommend to my middle-aged
friends who find the movie bewildering that they should read the book;
their teenage kids don't need to."

6 Answers
6

The other answers already provide some very interesting insights on the matter, which largely amount to the fact that the movie is more or less intentionally left ambiguous and without a clear interpreation, which is also what makes it so fascinating and engaging.

I'd like to amend this here with Wolfgang Schmitt's interesting analysis of the movie (unfortunately not available in English). He goes down pretty much the same line and concludes that the movie in its lack of a clear explanation is basically a statement about the universality, non-representationality and non-explainability of art itself. He says that the movie is actually deliberately constructed in a way to prevent any secondary interpretations of it and to prevent an easy-out approach to explaining it (translated by me):

...Why is the structure of 2001 so complicated? That is because it is actually layed out that way, it's constructed in a way so that it can't be explained with one working interpretation. [...] Every narrative thread we follow is lost in the void at some point. Even the episode about HAL where we think: "yes, that's quite tangible", we can say we have artificial intelligence and human intelligence and one will prevail over the other. Even if we realize this, the movie stops at this point and continues at a totally different point. Stanley Kubrick intentionally wants to lock himself from the secondary [...] He is concerned about the primary. And he wants to lock himself from something else, maybe even stronger than from those interpretations: from the nowadays so popular arts education that thinks you just need to take a work from its pedestal a little and suddenly you can serve it to everyone, that thinks art is democratic, you have to "pick up the people where they stand". Exactly that is what Kubrick does not do. He locks himself and that's what makes the greatness of his work.

I think it's great that especially this movie is so popular, although it stands against all that what we generally expect from art nowadays. Especially we always think that the artwork has to communicate with the viewer [...] But Kubrick's work does not communicate, and that's already evidenced by the fact that there isn't hardly spoken a word in this movie [...] Because A Space Odyssey is first and formemost a movie about art, as there is a very radical artist standpoint assumed here. 2001, that's actually the manifest of Kubrick.

For this he choses the monolith as an example of a completely uncontextualizable and unexplainable piece of art but that still provokes a change, equating this to the 20th century art movements of suprematism and minimalism that understood art as only having itself as content:

Let's take a look at the most mysterious thing in the whole movie, the black monolith. What does it stand for? It appears in all spaces and all times over and over, and there's always change induced by its mere presence [...] But let's think about the art of the 20th century. Don't we find there also such art objects like this black monolith? When we think about the suprematism, about Kazimir Malevich and the "Black Square". That's basically beginning and end of art in one, the "Black Square" stands for the art, it seems to be the concentrate, it's concrete, it's a direct experience when he stand in front of this "Black Square", and at the same time we can't say for what it actually stands. Or let's think, for the movie is from 1968, about the minimalism which is especially popular in America [...] There we find such sculptures that are just there in the room but behind which there isn't a direct meaning so we can't say "this stands for that" like we know it from classical paintings. [...] When there is something in the room, we can usually contextualize it [...] But what when there's something that at first doesn't have any meaning and no connections to the other things in the room? Then we're really confused, our coordinate system doesn't work any more. That's what 2001 is about. The monolith is a work of art that locks itself from every interpretation and especially every education. And the closedness is grounded in the fact that the artwork only relates to itself. There is a nice quote from Kazimir Malevich and his thoughts about art:

The new art has placed the principle into foreground that art can only have itself as content. So we don't find the idea of something in it, but only the idea of art itself, its self-content. The inherent idea of art is the non-representationality.

Kubrick follows exactly this tradition.

And when now equating the movie itself to this monolith he says, that the movie demonstrates the universality and power of art that actually comes from its unexplainability and that the fact that we'll never completely grasp the movie is its advantage and gives it it's actual meaning.

We stand in front of Kubrick's movie like in front of the black monolith. [...] This monolith seems to change something. We can approach it, we can try to explain it, but we never really grasp it completely. There's hardly a movie that supports such a radical concept of art and that's so popular at the same time. Therein lies the art of Stanley Kubrick.

The film starts and ends with music but in a quite strange way, for before we see the intro [...] there is just nothing and we hear music [...] And also at the end it's the same, when the end credits are all over we still hear Johann Strauß' waltz melodies. What does this actually mean? When we look at the screen we actually see nothing, everything is black [...] One could also say the monolith was zoomed comletely close here, and from this monolith, this auratic work of art, everything is possible [...] So the music at the beginning and the end isn't a prelude and afterplay, but it's an approach to art that says "art is always there". [...] With 2001 Stanley Kubrick has created a movie about cinema itself. For from this place where you first see nothing everything is possible. And where does film start and where are its limits? Do we need a clear plot structure? No, that's clear from this movie. Do we even need images? Even that is questionable, alone the fact that there is something acoustical and we look in the direction of the screen already means we encounter an artwork [...] Kubrick said about this movie, and he didn't say much about his movies, "In 2001 the message is the medium itself", so it's about the movie itself. 2001 itself is the monolith, an erratic work of art we can only approach but never completely understand, but this work or art changes us. And that's what great art can do.

I always thought that the ending of 2001 was open in the following sense.

The film narrates of two main steps in human evolution, both triggered by alien intervention using the monolith as an inductive device.

The first occured in the far past, where ape-like creatures were tranformed in homo sapiens through an enhancement of their intelligence.

We have to expect that a similar enhancement characterizes the second step, located in the near future of the year 2001, when the dying body of David Bowman is transformed into a newborn.

Now, the first act of the first creature with the intelligence of a homo sapiens was to concieve a new powerful weapon and eventually use it against other, formerly equal, men to his own advantage. All human technology (that's underlined by the famous bone-to-starship jump sequence) derives from that first killing tool.

So, what will be the first act of the newborn starchild who is already so powerful to be able to return to Earth almost instantly? What will be his (?) feelings for the humans on Earth?

There was actually originally supposed to be a voice-over in the ending, and for unknown reasons it was cut. The original script can be viewed here. Hopefully that helps you align the meaning of the visuals with the narrative in the book and in your mind.

To me the brilliance of 2001 lies in that it's a perfect storm of brilliant individual parts. It has jaw-dropping cinematography and SFX (that holds its own even in this CGI rich era), an iconic soundtrack, amazing writing (HAL remains one of the most chilling adversaries in movie history), and an intentionally ambiguous ending.

When you finish watching 2001 you are left with more questions that answers. Now, if you read up on anything related to the greater cosmos around us you'll get the same feeling of delicious bewilderment. That is part of the message of 2001 (the book or the movie); mankind as a tiny insignificant speck trying to find its place in a vast universe it can't begin to fully comprehend.

You don't need to read the book (although I do recommend that you do) to enjoy the movie on its own merits, as there isn't any truly "canonical" explanation to the events of the movie. You're supposed to feel lost at the end. Which, in my opinion, the movie does better than the book.

I have to say the book was pretty dull and one of the few movie treatments, where the movie is superior to the book, also worth stressing as it has been done elsewhere that the book was published after the film so some things are not quite the same. The book explains the ending better than the film but is a far less satisfactory experience. For me the movie is pure cinema magic, no one now would attempt to make a movie like this, expensive SFX, little action, heavy science, no talking, abstract ending, real genuine piece of art in my opinion.
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EdChumMar 14 '12 at 21:06

In the book, the moon was a portal which lead to a series of more portals. A giant intergalactic highway. The space ship, lacking fuel, ended up colliding with a star, presumably killing Dave. He then wakes up in the white room, as seen in the movie, and becomes a "star-child". The reader now understands that the obelisks were placed by supreme "aliens," the same who were behind the white room and the portals.

The movie is faithful to the book, although it intentionally lacks narration explaining what's going on. Unlike the book, the movie is an allegory to a number of things, and shouldn't be interpreted as being simply an adaptation of the book, but much more. It's an artistic approach to a scientific novel.